1. Field of the Invention
The field of the invention is that of the broadcasting of digital data intended to be received notably by mobile receivers moving about in an urban environment, i.e. under conditions of multiple propagation (Rayleigh Process) generating phenomena of fading, and in the presence of parasites and jamming. More generally, the invention relates to the broadcasting of digital signals in channels assigned multiple paths, the characteristics of which vary in time.
The invention can be applied more particularly, but not exclusively, to the system of digital sound broadcasting known as the COFDM (Coding Orthogonal Frequency Division Multiplex), as described in the U.S. Pat. No. 4,881,241 of Nov. 14, 1990.
This system of digital broadcasting is based on the combined use of a channel coding device and a method of modulation by orthogonal frequency division multiplexing.
2. Description of the Prior Art
The modulation method proper of this prior art system can be used to resolve the problems related to the frequency selectivity of the channel. It consists in providing for the distribution of the constituent digital elements of the data signal in the frequency time space f-t, and in simultaneously transmitting sets of digital elements on a plurality of parallel broadcasting channels by means of a multiplex of frequencies using orthogonal carriers. In particular, this type of modulation makes it possible to prevent two successive elements of the data train from being transmitted at the same frequency.
The known encoding method seeks, on the whole, to enable the processing of the samples coming from the demodulator to absorb the effect of variations in amplitude of the received signal, due to the Rayleigh process. The encoding is advantageously a convolutive encoding, possibly concatenated with a Reed-Solomon type of encoding. The decoding may be advantageously a soft decision type of iterbi decoding.
In a known way, the encoded digital elements are furthermore interlaced (interleaved), in time and in frequency, so as to maximize the statistical independence of the channels with regard to the Rayleigh process and to the selective character of the channel.
The demodulation of the received signal may be differential or coherent.
The value of differential demodulation lies essentially in the simplicity of its implementation and in its absence of inertia after a deep fading. It is this approach that has been used to validate the general principles of the COFDM system.
In theory, coherent demodulation offers greater resistance to noise than differential demodulation, making it possible to obtain a gain in performance of about 3 dB. However, it can be clearly seen that, under the reception conditions specific to systems of broadcasting towards moving receivers in a disturbed environment, it is particularly difficult to extract a phase and amplitude reference from the modulation signal for each carrier of the multiplex. In the case of coherent demodulation, the error made in the estimation of the carrier therefore leads to a substantial deterioration in performance characteristics. This is especially true in the case of deep and fast fading encountered when the carrier frequency or the speed of the vehicle increases.
In other words, coherent demodulation, in principle, performs better than differential demodulation, but makes it necessary for the carrier recovery device to be capable of giving a good estimate of the frequency response of the channel at any instant.
There is a method, known from the patent application No. FR 90 01491 dated Feb. 6, 1990 (corresponding to the U.S. Convention patent application U.S. Ser. No. 07/648,899 filed on Jan. 31, 1991) for broadcasting with time-frequency interlacing: this method enables coherent demodulation in providing for the insertion, among the useful information elements to be transmitted, of reference elements of the value and position, known to the receivers, in the frequency-time f-t space.
The basic idea of this method therefore consists in using certain carriers, judiciously distributed in the time-frequency domain, as phase and/or amplitude reference pilot frequencies. That is to say, fixed words, acting as amplitude and/or phase references at reception are inserted, at predetermined locations, among the data elements to be transmitted.
Thus it is possible, by interpolation, to determine a phase and amplitude reference for each of the digital elements, and hence to carry out a coherent demodulation.
More precisely, the estimation of the response of the channel is obtained by an interpolation filtration, either by circular convolution, as described in the already mentioned French patent application No. 90 01491 or by Fourier transform. The advantage of this latter approach is that, for equal quality, it needs a smaller number of operations than the former one.
However, it is seen that these methods do not provide satisfactory results in practice. Indeed, while the gain for the coherent demodulation as compared 7ith the differential demodulation is theoretically 3 dB when the response of the channel is perfectly estimated, in practice it is only 0.5 dB.
This poor result is due essentially to the fact that the estimation of the response of the channel is highly noise-infested, thus adversely affecting the quality of the interpolation.
The invention is designed notably to overcome this drawback of the prior art.